Exclusive production of pentaquarks in the scaling regime ∗

We investigate two exclusive reactions with a Θ + pentaquark in the final state: electro-production of a K meson on the nucleon, and K + scattering on a neutron target producing a lepton pair. These reactions offer unique opportunities to investigate the structure of pentaquark baryons at parton level. We discuss the generalized parton distributions for the N → Θ + transition and give the leading order amplitude for these processes in the Bjorken regime. Several experiments have reported evidence for the existence of a narrow baryon resonance Θ + with strangeness S = +1, whose minimal quark content is uudd¯ s [1]. Triggered by the prediction of its mass and width in [3,4], the observation of this hadron promises to shed new light on our picture of baryons in QCD. A fundamental question is how the structure of baryons manifests itself in terms of the basic degrees of freedom in QCD, at the level of partons. This structure at short distances can be probed in hard exclusive scattering processes, where it is encoded in generalized parton distributions [5,6]. In Ref. [7] we introduced the transition GPDs from the nucleon to the Θ + and investigated electroproduction processes where they could be measured, hopefully already in existing experiments at DESY and Jefferson Lab. The crossed channel, where a timelike virtual photon is produced together with the pentaquark, might be studied at intense kaon beam facilities presently under discussion [2].